1. Field of the Invention
This invention relates to two electrode welding method, and more particularly it relates to a novel two electrode welding method suitable for use in welding copper as in joining a commutator bar of an electric motor to a coiled conductor.
2. Description of the Prior Art
Hitherto, a commutator bar and a coiled conductor of a rotary electric machine, such as a motor or generator, have been formed of copper or copper alloy material having good heat conductivity. When these parts are joined together, it has been usual practice to rely on brazing, plasma welding or TIG welding. In plasma welding, gas is ejected at high speed from a nozzle and sagging tends to occur. With members to be welded which are large in thickness and the weld length being small, brazing or one electrode TIG welding is preferable. However, in carrying out brazing and one electrode TIG welding, it is necessary that the members to be welded be heated to 200.degree.-400.degree. C. before welding is performed. In addition, there are the disadvantages in TIG welding that since it is impossible to perform continuous welding, it is necessary to take a long period of time for carrying out welding, and that since the members to be welded are heated in the entirety over a prolonged period of time, the weld zone shows a reduction in hardness, the grain becomes coarse, and blowholes are formed, thereby greatly reducing the quality of the product.
In order to carry out welding as desired, the use of a two electrode TIG welding process will be contemplated. The two electrode TIG welding method which is described in U.S. Ser. No. 99,848 (1979) that is a prior application of the present application and which is disclosed in U.S. Pat. No. 3,627,974 has two electrodes or a leading electrode and a trailing electrode from which arcs are generated for forming a molten metal pool by utilizing the heat of the arcs, to thereby form a deposited metal.
The two electrode TIG welding process as described hereinabove, however, might be disadvantageous depending on the shape and configuration of the weld zone. For example, when a commutator riser of an electric motor is welded to a coiled conductor, mica segments adjacent the riser would suffer overheating and burn and gas would be ejected as the riser and the coiled condutor are melted by the heat of the arc generated by the leading electrode. The gas would be incorporated into the shield gas, such as argon, ejected through a torch nozzle and make the arcs unstable. Moreover, the gas would be dissolved into the weld zone under the pressure of the arcs and mixed into the molten metal, thereby causing a multiplicity of blowholes to be formed in the deposited metal.
This phenonema has a delicate relation to the value of the welding current, the volume of the argon gas, the welding speed, and the shape of the torch nozzle. In a two electrode TIG welding process of the prior art, difficulties have been faced with in preventing the production of gas from combustion of mica segments and its incorporation in the deposited metal.
The presence of blowholes in the weld zone markedly reduces the mechanical strength of the weld zone and the region in its vicinity. Also, this reduces the cross-sectional area of the depositee metal, so that the commutator riser shows a rise in its resistance value and its temperature rises to a high level when a current is passed thereto.
To obtain complete joining of the riser and coiled conductor formed of copper or a copper alloy requires preheating to a temperature over 400.degree. C. In view of the structure and performance of the commutator, however, the commutator bars shown a reduction in strength and hardness when heated to a temperature over 200.degree. C., and the commutator as a whole undergoes deformation, so that the commutation function of the commutator is greatly impaired. Thus it has hitherto been difficult to subject the commutator and coiled conductor to preheating to a satisfactory degree at high temperature.